Vermiculite is an odorless, fireproof mineral substance. Along with another naturally occurring substance, perlite, it's often added to potting soil.

Vermiculite provides many benefits. These include -

• Enhances drainage: Vermiculite soaks up water like a sponge. It holds that water until the soil starts to dry out then releases it. This is good for moisture loving plants but not so good for plants that like dryer soil.
• Adds permanent soil conditioner: Unlike compost, which eventually breaks down into the soil, vermiculite doesn't break down. Compost does add vital nutrients, but if you need to permanently improve drainage, vermiculite is a great solution.
• Slightly raises pH: Vermiculite is pH neutral, around 7.0 or so, but because of some compounds found within it, it can slightly raise pH, a plus for acidic soils.
• Makes other minerals available: Vermiculite naturally reacts with soil compounds and makes available other nutrients such as calcium, potassium and magnesium.
• Vermiculite can be used in the storing of bulbs and root crops to help prevent rot.  Remove excess soil from bulbs and tubers and dry outside.  Store in containers in a cool dark place, covering with Vermiculite.   The absorptive power of vermiculite acts as a regulator that prevents mildew and moisture fluctuation during the storage period. It will not absorb moisture from the inside of stored tubers, but it does take up free water from the outside, preventing storage rot.

Grades:

• Fine:       0-5mm   (Ideal For seedling soil mix)
• Medium: 0-10mm (Ideal for potting soil mix)
• Coarse:  0-15mm (Ideal for storing tubers and other root vegetables, Large pots, Fireplace Insulation)

Perlite v Vermiculite

• Perlite is better than vermiculite at improving aeration. Including perlite will create a mix that has more macroporosity.  That is the spaces between perlite particles are big enough to allow water to drain through them whilst also allowing for water-retention and the effects of capillary action in small surface crevices.
• Vermiculite is a better water-holder. Vermiculite is more sponge-like and has a higher water holding capacity than perlite. Perlite particles can only hold moisture in the small bubbles on their surface
•  For seed germination, where near-permanent moisture is required, vermiculite is ideal. Seeds germinate and grow much better in moist conditions and for this reason vermiculite is a good medium for germination.
• Perlite is better for cuttings due to to its high macroporosity. Cuttings tend to benefit from having access to lots of oxygen as well as water
• As an addition to a potting mix: vermiculite for increasing water-holding capacity, perlite for improving drainage (air porosity).